In U.S. Pat. No. 5,036,351, a method and a device are known for producing images on an information carrier, e.g., paper, by means of electrostatic fields and using an electrode unit between a toner carrier member, so-called developer, and a back electrode.
The electrode unit can be formed as a woven net, consisting of electrodes crossing each other. Net meshes are arranged between the electrodes, through which toner particles are attracted from the toner carrier member towards the back electrode. By connecting electrodes, surrounding the meshes of the net, to different voltages, passages for the toner particles via meshes are opened and closed.
The electrode unit can also consist of a thin substrate, arranged with pervious apertures, which are surrounded by the electrodes, so-called ring electrodes. Such electrode units are described in U.S. Pat. No. 5,121,144.
Another type of electrode unit is shown, for example, in UK 2 108 432, in which two electrode layers are arranged on each side of a conveyer, and the electrodes surround pervious holes to modulate particles from a toner carrier member towards a back electrode through the holes.
FIG. 1 shows a schematic view of a printer unit 10 according to SE 9000031-6 or SE 8704883-1. The toner particles 11, which are adhered onto the toner carrier member by means of magnetic or similar forces, are transported from the toner carrier member 12 towards a back electrode 15 via the electrode unit by means of electrostatic fields. One condition to obtain transportation of the toner particles from the toner carrier member 12 onto an information carrier 13 with a good result is that the distance l.sub.k between the toner carrier member 12 and the electrode unit 14 is so short that the strength of the field that transports the toner particles is as strong as possible. The transportation field between the toner carrier member 12 and the back electrode 15 is obtained, for example, by connecting the toner carrier member to ground (0 V) and the back electrode to 1.5 kV. By connecting the electrodes 16 in the electrode unit 14 to a variable control voltage V.sub.0, for example 300 V, passages through apertures 17 are generated. The size of these passages can be varied to allow the transporting field to pass entirely, partly, or not at all through the apertures and convey toner particles in the direction towards the back electrode 15 and onto an information carrier 13 placed between the back electrode and the toner carrier member 12, such as a paper sheet.
The toner carrier member 12 is rotatably arranged in a toner container (not shown) and attracts toner particles by means of, for example, magnets (not shown) provided inside the toner carrier member. The toner particles 11 attracted onto the toner carrier member are leveled to a layer on the surface of the toner carrier member, which can include conducting or semiconducting material.
The toner carrier member is typically formed as a roller with a circular cross section, and the electrode unit is generally formed flat. Due to the cylindrical form of said roller, the distance l.sub.k from the surface of the roller to the apertures 17 varies. The variation is designated with .DELTA.l.sub.k. For example, the distance from the toner carrier member to the apertures A2 and A3 is shorter than to the apertures A1 and A4, with respect to a perpendicular line (not shown) from the center of the roller to a point between apertures A2 and A3.
The schematic graph of FIG. 2 shows the relationship between the distance l.sub.k and the electrostatic field E for attracting the toner particles in a direction towards the back electrode. Variations in .DELTA.l.sub.k result in variations in the electrostatic field E, which in turn causes variations in the number of toner particles which are attracted towards the surface of the information carrier 13. Said variations in the amount of the toner particles affect the printing quality and cause undesired variations in the produced image.
Another problem that may occur is at color intensive prints. This problem, so-called "white line noise," which is best illustrated in FIGS. 1 and 3, results in appearance of lighter lines in the image, because the toner particles on the toner carrier member are not enough for all apertures of the electrode unit.
In FIG. 3 the arrow shows the rotation direction of the toner carrier member. If, for instance, apertures A1-A4 are opened in numerical order, some portion of the toner particles 18 will be transported onto the information carrier 13 in a consecutive order, then the aperture A1 will receive the most of the toner, while the subsequent apertures receive lesser and lesser amounts. At aperture A4 the amount of the toner may be so much lesser that a deterioration in the printing quality occurs.
Another problem is that the toner particles "see" the conductive electrodes 16, as a source of the fields, which attract the toner particles. Lines illustrated with dots and dashes show the areas A'1-A'4, B'1 and B'2, which affect the toner particles on the toner carrier member. The shadowed areas show how the toner particles will be applied, i.e., will be transported onto the information carrier. It appears from the figures that B1 will also affect the printing area of A4.
Yet another problem which may arise is the so-called "curtain effect," where a large area image has reduced blackness at the beginning of the printing, followed by normal blackness after a short distance. This problem occurs when several adjacent electrodes 16 are energized at the same time to produce a large area image. The combined electric field of several adjacent energized conductors 19 is strong enough to attract the toner particles from the toner carrier member to the conductors. Diversion of those toner particles reduces the number of toner particles deposited on the paper, causing a less black image. The diversion of the particles soon becomes saturated, allowing normal black printing to occur for the remainder of that image portion. At the end of printing that image portion, the toner particles are attracted back onto the toner carrier member.